Fix build error when building a shared libunistring on Android. * tests/uninorm/test-nfc.c (n): Don't define on Android. (main): Add 'volatile', to defeat a GCC optimization that would eliminate the reference. * tests/uninorm/test-nfd.c (n): Don't define on Android. (main): Add 'volatile', to defeat a GCC optimization that would eliminate the reference. * tests/uninorm/test-nfkc.c (n): Don't define on Android. (main): Add 'volatile', to defeat a GCC optimization that would eliminate the reference. * tests/uninorm/test-nfkd.c (n): Don't define on Android. (main): Add 'volatile', to defeat a GCC optimization that would eliminate the reference.